Two years on, Huygens data still delivering

Titanic probe

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Two years ago, a small spacecraft fell through a turbulent atmosphere to land on the surface of an alien world a billion or so kilometres from Earth. Since then, the data collected during its relatively short descent has continued to delight and amaze space scientists, amateur astronomers, and anyone with a passing interest in alien worlds, so it seems churlish to let the anniversary pass unnoticed.

Huygens, named for Christian Huygens, the Dutch scientist and pendulum clock patenter who first discovered Titan back in 1655, was part of a collaborative mission to explore Saturn and its system of moons and rings.

The probe hitched a lift aboard Cassini for the seven years of its journey, before separating and heading towards Titan.

The journey might have been long, but Huygens' active life was very brief. The probe was only able to return data to Earth for as long at it could see Cassini as it relied on the NASA craft to relay the signal home. For all we know it's still there, gabbering away to itself, like some kind of cosmic-nutter-on-the-bus.

It took the craft two and a half hours to fall to the surface. On its way down it was able to build a profile of the atmosphere. It also sent back surprising data about the wind, which unexpectedly dropped away at 60km.

On landing, it measured its impact, revealing much about the kind of surface it had hit. The probe also carried a gas chromatograph and mass spectrometer, which could record details of the atmosphere at the surface. This confirmed the presence of methane and of two isotopes of argon.

"When you put all the data together we get a very rich picture of Titan," says Athéna Coustenis, Observatoire de Paris, France. "The Descent Imager/Spectral Radiometer (DISR) pictures were an enormous surprise. We had expected a much smoother landscape."

The presence of the two argon isotopes was particularly important because it suggested something about the history of the world. Argon 40, for instance, suggests the moon is still alive in its interior. Researchers suggest the moon has an insulating layer of ice and water which keeps its interior warm. This internal heat then escapes periodically in the form of so-called cryo-vulcanism.

The presence of the other isotope of argon, argon 36, might indicate when the moon formed. Parisian researcher Daniel Gautier suggests it shows the moon formed after Saturn, at a time when the infant solar system had cooled to around 40K.

And the data is still providing more information for researchers, even two years after the event.

Already in 2007, combining the data from Huygens with new observations from Cassini, the team has been able to confirm that there are lakes of liquid methane on the moon's surface.

Francesca Ferri from the Università degli Studi di Padova sums up the mission: "There are lots of surprises still to come from this data." ®